[Metricbeat] Check if fields in DBInstance struct is nil

[kaiyan-sheng]

When metricbeat is running with aws rds metricset enabled, create a new rds instance and this error showed up:

2019-08-20T15:03:02.210-0600    ERROR   runtime/panic.go:82     recovered from panic while fetching 'aws/rds' for host ''. Recovering, but please report this.  {"panic": "runtime error: invalid memory address or nil pointer dereference", "stack": "github.com/elastic/beats/libbeat/logp.Recover\n\t/Users/kaiyansheng/go/src/github.com/elastic/beats/libbeat/logp/global.go:105\nruntime.gopanic\n\t/usr/local/go/src/runtime/panic.go:522\nruntime.panicmem\n\t/usr/local/go/src/runtime/panic.go:82\nruntime.sigpanic\n\t/usr/local/go/src/runtime/signal_unix.go:390\ngithub.com/elastic/beats/x-pack/metricbeat/module/aws/rds.getDBInstancesPerRegion\n\t/Users/kaiyansheng/go/src/github.com/elastic/beats/x-pack/metricbeat/module/aws/rds/rds.go:165\ngithub.com/elastic/beats/x-pack/metricbeat/module/aws/rds.(*MetricSet).Fetch\n\t/Users/kaiyansheng/go/src/github.com/elastic/beats/x-pack/metricbeat/module/aws/rds/rds.go:88\ngithub.com/elastic/beats/metricbeat/mb/module.(*metricSetWrapper).fetch\n\t/Users/kaiyansheng/go/src/github.com/elastic/beats/metricbeat/mb/module/wrapper.go:244\ngithub.com/elastic/beats/metricbeat/mb/module.(*metricSetWrapper).startPeriodicFetching\n\t/Users/kaiyansheng/go/src/github.com/elastic/beats/metricbeat/mb/module/wrapper.go:212\ngithub.com/elastic/beats/metricbeat/mb/module.(*metricSetWrapper).run\n\t/Users/kaiyansheng/go/src/github.com/elastic/beats/metricbeat/mb/module/wrapper.go:199\ngithub.com/elastic/beats/metricbeat/mb/module.(*Wrapper).Start.func1\n\t/Users/kaiyansheng/go/src/github.com/elastic/beats/metricbeat/mb/module/wrapper.go:138"}

When a new RDS instance is still in creating state, it has an arn but not AvailabilityZone yet. That's why this invalid memory address or nil pointer dereference error shows up.

closes #13037

[kaiyan-sheng]

jenkins, test this please